How
random is our winter weather?

Climate scientists
map out a hemisphere-wide web of connections

by Bob Henson

Although forecasters wrote the lines well in advance, the winter of
200203 strayed from its script across the United States. Extended
outlooks, based on the presence of a weak to moderate El Niño,
had called for wet weather along the nations southern tier and
mild, dry conditions to the north and west.

In the Denver areas heaviest winter storm in 90 years, some
foothill locations got upwards of 200 centimeters (79 inches) of snow
on 1719 March. However, most of the intermountain West remains
in a severe to extreme drought. (Photo by Carlye Calvin.)

Some of these trends played outMichigan had its driest winter
on record, for instancebut unforeseen events stood out in sharp
relief. The mid-Atlantic endured one of its wettest, coldest winters
in decades, with a parade of snowstorms marching up the East Coast.
Instead of the expected north-to-south temperature contrasts, the nation
was split from east to west, with record warmth across the Rockies and
persistent cold east of the Mississippi.

These surprises point to the limits of the El Niño/Southern
Oscillation (ENSO) as a long-range forecast tool. A flurry of research
since the mid-1990s has explored another vast system, involving both
the Atlantic and Arctic, that influences winter weather across the Northern
Hemisphere. The full extent of this system and its physical mechanism
are still matters of debate, but according to scientists at NCAR and
elsewhere, it could serve as a useful tool for predicting weather swings
within a given winterand perhaps the long-term evolution of northern
climate.

Bridging the Arctic and
Atlantic

Two concepts, with considerable overlap, lie at the heart of this
research.

The North Atlantic Oscillation (NAO) is a seesaw in atmospheric pressure
that helps direct the flow of winter storms from eastern Canada to Europe.
A positive NAO index means the contrast between high pressure over the
Azores and low pressure in the far north Atlantic is stronger than normal.
This usually drives mild Atlantic storms into the bulk of Europe but
keeps the Mediterranean on the dry side (see graphic, pg. 5). A negative
NAO index denotes a weakened pressure pattern that opens the door to
cold, dry Arctic intrusions into northern Europe and wet, slow-moving
systems across the south.

The Arctic Oscillation (AO) involves not just the Atlantic but the
full band of winds that encircles the North Pole at about 55°N.
When this vortex tightens (a positive AO index), it tends to lock Arctic
air over the pole. When the vortex loosens (negative AO), frigid air
masses can dive more easily into North America, Europe, and Asia. This
pattern is also called the Northern Annular Mode, in parallel with its
Antarctic counterpart, the Southern Annular Mode.

NCARs Clara Deser summarized the AOs climate effects in
February for the annual meeting of the American Association for the
Advancement of Science. She noted that the Arctic vortex was weaker
than usual early this winter, a switch from the strongly positive AOs
noted since the 1980s. This might have helped lead to the bitter cold
that swept the eastern U.S. and Europewhich seemed all the more
bracing after the mildness of preceding winters.

The Arctic Oscillation has strengthened in recent decades, which
is contributing to unusual warmth over the Northern Hemisphere land
masses, says Deser. Global climate models are on to this trend
as well: as greenhouse gases increase, the models tend to produce a
positive AO.

However, Deser stresses that the AO/NAO is an intrinsic aspect of
climate. You dont need changes in greenhouse gases or sea-surface
temperature or other external factors to find this pattern. It exists
because of the nature of the atmosphere itself.

Medieval discoveries

NCARs James Hurrell is lead editor of the most detailed book
to date on the NAO (see photo). Hurrell says the cycle is one
of the oldest known world weather patterns. Nearly a thousand
years ago, Vikings recorded a classic aspect of the NAO: severe winters
tend to strike Greenland in tandem with mild winters in Denmark, and
vice versa

.

All about the NAO Published by the American Geophysical
Union last winter, "The North Atlantic Oscillation: Climatic Significance
and Environmental Impact" is the most thorough treatment of the
NAO to date. NCARs James Hurrell (above) edited the book with
Yochanan Kushnir and Martin Visbeck (Columbia University) and Geir Ottersen
(University of Oslo). The books twelve review papers cover the
NAOs history, predictability, ecosystem impacts, and other pertinent
research. (Book cover art courtesy AGU; photo by Carlye Calvin.)

In the 1920s, pioneering climatologist Sir Gilbert Walker identified
both the Southern Oscillation and the NAO. But unlike the oceanic metronome
that gives ENSO its semi-regular beat, the NAO is a much more irregular
cycle, less constrained by the ocean below it.

Researchers in the 1940s and 1950s, including Carl-Gustav Rossby and
Edward Lorenz, focused attention on the west-to-east flow that girdles
the middle and high latitudes. This provided a new way to think of the
NAO: a variation in the strength of the westerlies across the Atlantic.
Their work also paved the way for the current AO/NAO concepts, whose
leading proponents include David Thompson (Colorado State University)
and Michael Wallace (University of Washington).

Interest in the AO and NAO exploded in the late 1990s with the arrival
of improved ocean-atmosphere models and more sophisticated statistical
analyses. According to the Institute for Scientific Information, the
number of papers that mention the NAO in either title or abstract leapt
from 11 in 1996 to 179 in 2001.

What drives the cycles?

Scientists havent yet agreed on how the AO and NAO do or dont
differ. In my view, the AO and NAO are different interpretations
of the same phenomenon, says Thompson. I think the debate
over this pattern attests to the absence of a unique theory for its
existence in the first place. In spite of the uncertainty, some
potential forecast tools are starting to surface.

 Weather from the stratosphere. In 1999, Mark Baldwin and Tim
Dunkerton (Northwest Research Associates) discovered that large changes
in the stratosphere often precede similar changes in the Arctic Oscillation
at the surface. Thompson and colleagues have since found that these
stratospheric anomalies tend to precede extended cold snaps at ground
level throughout much of the Northern Hemisphere. In related research,
Thompson and Susan Solomon (NOAA) noted that similar downward-propagating
stratospheric events are observed in the Southern Hemisphere, possibly
initiated by the ozone depletion observed there since the 1980s.

Once a stratospheric climate signal hits the lower atmospherewhich
can take a few days to several weeksthe effects may persist for
a month or more. Even so, its not clear whether the stratosphere
was responsible for this winters cold blasts, says Thompson. This
last winter there was some variability in the stratosphere, but nothing
quite as dramatic as weve seen in other years, he says.

Although many other factors influence our winter weather, Thompson
is optimistic about using stratospheric data as a sign of weather changes
to come in the 30- to 60-day time frame. The evidence suggests
that the usefulness of such forecasts may prove to be roughly comparable
to the usefulness of forecasts based on ENSO, he says.

 The Pacific influence. A peculiar sequence of high- and low-pressure
centers straddling the midlatitudes was part of this winters weather
picture. NCARs Grant Branstator was excited to see this pattern,
because it supports a finding that emerged from his experiments with
the NCAR Community Climate System Model. The idea is that energy in
the northern midlatitudes gets channeled through the strong subtropical
jet that roars across the globe around 30°N. One of the most prevalent
modes of this channeling, says Branstator, is a pattern that looks remarkably
like the AO/NAO.

Intrigued by this winters pattern, Branstator tried to reproduce
it in the model by warming various parts of the tropical Pacific. The
closest resemblance to the 200203 pattern occurred when the models
ocean was warmer than average near the International Date Line. Thats
close to where this years El Niño warming was centered.

As these things go, this is a pretty good match, Branstator
says. He points out the likely presence of natural variation in the
mix, but hell continue to examine the possible link between the
tropical Pacific and the AO/NAO.

Further west, the role of the western tropical Pacific and Indian
oceans in North Atlantic climate has been examined by modeling work
by NCARs Hurrell and Martin Hoerling (NOAA). The two have found
positive AO/NAO trends associated with rising sea-surface temperatures
in both basins.

The other ocean

Thanks to El Niño, the tropical Pacific has long been a hot
spot in climate modeling. Only recently has the tropical Atlantic been
getting the attention it deserves, says Christophe Cassou, who is completing
a two-year visit to NCARs Climate and Global Dynamics Division.
To look at the AO/NAO system fruitfully, says Cassou, you have to analyze
the short-term regime shifts that can get lost inside long-term averages.

Cassou will return this autumn to Frances European Center for
Research and Advanced Training in Scientific Computation. He and colleagues
at NCAR and CERFACS have been using a tool called cluster analysis to
get at the distinct climate modes that predominate across the North
Atlantic. Along with the positive and negative NAO regimes, theyve
found two other patterns at work over the last century, with some spatial
overlap among the four. Only in the last 20 years has the positive NAO
regime overwhelmed the other three patterns.

Ocean-atmosphere models are helping to relate these patterns to the
Atlantic itself, says Cassou. But much work remains to be doneincluding
work on the models themselves. The basic climate features of both
the tropical and extratropical Atlantic are poorly simulated in most
of the models, he says. Why? Nobody knows precisely.

With so many unanswered questions about Northern Hemisphere climate,
Cassous reply may resonate for some time to come.